The use of a diaphragm, formed of a flexible web carrying a heating element, for the application of pressure and heat to bond heat bondable material is disclosed in my U.S. Pat. No. 3,964,958, issued June 22, 1976, which is hereby incorporated by reference and hereinafter referred to as "said patent."
In said patent, a pressure chamber is formed by a cavity closed by a diaphragm. The materials to be bonded are supported on a supporting surface and the diaphragm is brought into contact with the materials over an area larger than the intended bond area by flexing the same convexly by regulated pressure applied by fluid in the pressure chamber. A heating element is carried by the diaphragm inside the pressure chamber cavity, on the non-material engaging side thereof, to apply bonding heat to the materials through the diaphragm. The configuration of the heating element defines the bonding configuration.
The system of said patent eliminates extrusion of the bond area which is inherent in typical die bonding systems. The strength of the bond approaches that of the material itself. The heating element employed is preferably a thin ribbon bonded to the web and is compatible with those bonding techniques known as resistance and impulse bonding, as well as that variously known as dielectric, electronic, high frequency or radio bonding.
In implementing the invention of said patent, it has been discovered that some applications may require a more precise heat control in the bond area and adjacent areas. For example, while a complete bond having the strength of the material in which it is formed is easily accomplished with the system of said patent, a more precise heat control will allow the strength of that bond to be more readily controlled so as to provide bonds that will pull apart with a predetermined force less than the strength of the materials. Such a bond is desirable in many packaging applications. Additionally, by precisely controlling the temperature in the bond area and adjacent areas, the bond width is precisely controlled and is affected very little by the duration of heat application. Also, it has been found that making electrical contact with the heating element at a point where it is bonded to the web increases the mass of the heated components at that location and alters the heat pattern that would otherwise be produced by the heating element. Further, the system of said patent requires careful material selection such that the coefficients of expansion of the heating element and web are as compatible as possible to minimize the forces generated upon heating of the heating element and expansion of the heating element and web.